US9307338B2 - Upmixing method and system for multichannel audio reproduction - Google Patents

Upmixing method and system for multichannel audio reproduction Download PDF

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US9307338B2
US9307338B2 US13/923,608 US201313923608A US9307338B2 US 9307338 B2 US9307338 B2 US 9307338B2 US 201313923608 A US201313923608 A US 201313923608A US 9307338 B2 US9307338 B2 US 9307338B2
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audio
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US20140010375A1 (en
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John Usher
Antonio SOLE
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Dolby International AB
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S5/00—Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation 
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S1/00—Two-channel systems
    • H04S1/002—Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S5/00—Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation 
    • H04S5/005—Pseudo-stereo systems, e.g. in which additional channel signals are derived from monophonic signals by means of phase shifting, time delay or reverberation  of the pseudo five- or more-channel type, e.g. virtual surround
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S2400/00—Details of stereophonic systems covered by H04S but not provided for in its groups
    • H04S2400/05—Generation or adaptation of centre channel in multi-channel audio systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S2400/00—Details of stereophonic systems covered by H04S but not provided for in its groups
    • H04S2400/07—Generation or adaptation of the Low Frequency Effect [LFE] channel, e.g. distribution or signal processing

Definitions

  • the present invention relates generally to signal processing for audio applications and more specifically to a novel and improved audio upmixer and method for upmixing stereophonic audio channels.
  • These multichannel implementations have also evolved to include “surround-sound” effects.
  • Such surround-sound loudspeaker audio systems are today found in theatres, music auditoria, automobiles, and domestic theatre and computer systems, amongst others.
  • these implementations typically comprise a wide variety of individual full-range loudspeakers and sub-woofers, each with their own sound characteristics and input/output responses.
  • Audio upmix, or upmixer systems have been proposed in order to effectively upmix N original audio signals into M upmixed audio signals, where M>N. For instance, systems exist which generate at least two surround audio channels. Other prior art systems produce two surround channels which detect hard-panned sources and ensure that voice signals will always be located in the front channels even if they exist in only one input channel.
  • upmixing systems for home or professional theatre systems are usually configured to generate 3 front loudspeaker signals, 2 surround signals, and a low frequency effects, LFE, or subwoofer, signal to drive a sub-woofer loudspeaker, as represented in FIG. 1A .
  • the 3 front loudspeaker signals are normally used for outputting all sound types, including voice, the 2 surround signals for producing ambient sounds and the LFE subwoofer signal is used to generate low frequency special effects.
  • This combination results in an enhanced experience for the end user due to the different sound components being generated in the different loudspeakers.
  • the sound imagery is enhanced because sound images are located around the listening area, giving a more natural enveloping imagery compared with reproduction on two frontal loudspeakers.
  • Matrix decoding is a type of adaptive or non-adaptive audio upmixing whereby a higher number of output audio signals (e.g. 6 for a 5.1 system) is decoded from a smaller number (typically 2) of input signals.
  • systems comprising non-matrix coding and decoding also exist.
  • phase inversion mixing is a very common audio technique used in music and film audio production to give a wide spatial imagery.
  • phase inverted input signals are normally summed, and since the out of phase signals cancel each other out, no signal is present in the LFE signal. Therefore the desired sub-woofer effect is not achieved.
  • a further disadvantage of existing systems is that sound components originally only present in one input channel are generated as output also in the centre channel, therefore producing a non-realistic outputsound image. For instance, consider a musical audio signal corresponding to a recorded musical instrument present on only the left input channel. If the upmixed centre channel is generated by summing the input left and right channels, then this upmixed centre channel will also contain the recorded musical instrument signal. This is an undesirable effect as it should only be perceived on the left when auditioned: that is, the spatial sound image quality of the auditioned upmixed signal will be poor.
  • time-smearing Another effect which audio signal processing equipments need to take into account is time-smearing. It is very common for music recordings, or speech recordings, from live conferences, or with live dialogue, in films and television, to use more than one microphone for the recording. Each microphone is normally physically positioned at different corners of the room. In this scenario, the sound being recorded happens to be physically closer to one microphone more than the others resulting in signals containing audio generated time-delay effects, due to the fact that the sound arrives in one microphone before the other. This effect is termed time-delay panning or time-smearing.
  • the resulting summed signal will contain a time-smeared signal, or a signal with a temporally smeared image, which results in reduced sound quality due, in part, to out-of-phase sound artefacts.
  • This effect can be readily understood if the signal to be recorded is simply a “click” sound. Since the click arrives in one channel before the other, then if a non-zero gain is applied to one or both channels and the result is summed, then two clicks will appear in the resulting summed channel. Again this results in a poor reproduction of the original sound image.
  • an audio signal enhancing device and a corresponding method of enhancing stereophonic signals, is provided which generates an enhanced signal with improved spatial sound image quality.
  • an improved processing of the input signals is provided resulting in final centre channel and at least one LFE sub-woofer channel wherein the problems and disadvantages of the prior art are resolved.
  • the result is a centre and LFE signal that contains a stable, non time-smeared image with a high quality natural-sounding fidelity.
  • a pair of audio signals is automatically upmixed for optimum reproduction via 3, or 5 or 7 full-range loudspeakers in combination with at least one 1, and even up to 3 sub-woofer signals.
  • the upmixing method of the invention is tailored for high-quality low-latency audio signal processing for voice, music and film soundtrack audio sources.
  • an audio signal enhancing device for enhancing a stereophonic input signal comprising two audio signals to generate at least one enhanced signal.
  • a method of enhancing a stereophonic input signal to generate at least one enhanced signal is provided.
  • a centre channel generation device and a corresponding method, for generating a centre channel signal from a stereophonic input signal comprising two audio signals is provided.
  • a low frequency effects LFE subwoofer signal generation device and a corresponding method, for generating a subwoofer signal from a stereophonic input signal comprising two audio signals is provided.
  • audio signal upmixer and a corresponding method, for generating at least three output audio signals from a stereophonic input signal comprising two audio signals is provided.
  • a computer program and a computer readable medium embodying the computer program, for performing the different functions of the different aspects and embodiments of the invention are provided.
  • the invention provides methods and devices that implement various aspects, embodiments, and features of the invention, and are implemented by various means. For example, these techniques may be implemented in hardware, software, firmware, or a combination thereof.
  • the processing units may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.
  • ASICs application specific integrated circuits
  • DSPs digital signal processors
  • DSPDs digital signal processing devices
  • PLDs programmable logic devices
  • FPGAs field programmable gate arrays
  • processors controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.
  • the various means may comprise modules (e.g., procedures, functions, and so on) that perform the functions described herein.
  • the software codes may be stored in a memory unit and executed by a processor.
  • the memory unit may be implemented within the processor or external to the processor.
  • FIG. 1A depicts an upmixing configuration of the prior art with 2 input channels and 6 output channels, or 5.1 output channels as it is also commonly known in the art.
  • FIG. 1B depicts details of the front channel processor of the prior art.
  • FIG. 2A depicts one embodiment of the present invention comprising details of the audio signal enhancing device for generating at least one enhanced signal from two audio signals.
  • FIG. 2B depicts another embodiment of the present invention comprising details of the front channel processor for generating a centre channel signal.
  • FIG. 2C depicts another embodiment of the present invention comprising details of the front channel processor for generating at least one, preferably three, subwoofer signals.
  • FIG. 2D depicts another embodiment of the present invention comprising details of the front channel processor for generating a centre channel signal and at least one, optionally three, subwoofer signals.
  • FIG. 3 depicts another aspect of the present invention, comprising details of the intermediate processor and the control processor.
  • FIG. 4 is a flowchart representation of a method of producing an intermediate signal according to an aspect of the present invention.
  • FIG. 5 depicts another aspect of the present invention, comprising details of the front channel processor for generating a centre channel signal.
  • FIG. 6 depicts a centre channel weighting curve according to an aspect of the present invention.
  • FIG. 7 is a flowchart representation of an aspect of the method of producing a centre channel signal according to an aspect of the present invention.
  • FIG. 8 depicts another aspect of the present invention, comprising details of the front channel processor for generating at least one low frequency effect subwoofer signal.
  • FIG. 9 is a flowchart representation of an aspect of the method of producing a at least one low frequency effect subwoofer signal according to an aspect of the present invention.
  • low frequency effect and “subwoofer” may be used in conjunction or interchangeably, as they both refer to the same feature, and can be summarised as “LFE”. Therefore the upmixed output signal may be expressed as low frequency signal or channel, LFE signal or channel, subwoofer signal or channel, LFE subwoofer signal or channel or low frequency effects LFE subwoofer signal or channel, or any other combination.
  • FIG. 1A shows a simplified schematic of a configuration of a 5.1 upmixing loudspeaker system of the prior art, wherein two original left and right input audio signals Lo 102 and Ro 104 are upmixed to 6 new signals.
  • Front channel processor 106 comprises, amongst other components, a centre channel processor 122 and an LFE channel processor 124 for generating the centre channel signal 112 and the subwoofer signal 108 respectively, as depicted in further detail in FIG. 1B . Therefore the front channel processor 106 processes the first input signal 102 and the second input signal 104 to yield at least four output signals, comprising a left 110 , a centre 112 , a right 114 , and a low frequency effects LFE 108 , or subwoofer, audio signal.
  • a rear channel processor 116 generates a pair of audio signals Ls 118 and Rs 120 that can be reproduced with rear “surround” loudspeakers. Since this invention does not relate to aspects of improving the surround-sound of prior art systems, the present disclosure does not further explain the details of the rear channel processor, or the rear channels. Those skilled in the art will realise that a workable surround-sound loudspeaker audio system includes a suitable combination of associated structural elements, mechanical systems, hardware, firmware and software that is used to support the function and operation of the surround-sound system.
  • FIG. 1 suffers from the problems that the front channel processor of the prior art, or processors when implemented as a plurality of elements, are so configured that a time-smeared centre channel signal is generated, and since out-of-phase components cancel each other out, no, or very little, significant LFE audio is generated at the output of the subwoofer loudspeakers. Hence the original signal is degraded by the audio processing of the prior art resulting in an uncomfortable experience for the end-user.
  • the present invention solves the problems of the prior art by proposing a front channel processor comprising a novel audio signal enhancing device, as an intermediate stage, common to both centre channel and LFE channel processing, for generating enhanced intermediate signals.
  • These enhanced signals are generated by taking into account the common sound components between the input signals, as the configuration of adaptive filters and delay lines, together with the dynamic setting of gain and filter coefficients, allows the correlated components of the input signals to be utilised and tuned according to the desired effect.
  • the enhancing device mixes only the loudest level (“level” here applies to a relative voltage magnitude, e.g. level in dBV) of two filtered signals so that out of phase signals are not cancelled, and the resulting level of the output channel is proportional to the original low frequency content in the original input signals. This is achieved in part by determining a pair of optimum filters that are used to filter two input signals so that when summed, the resulting signal will not contain time-smearing and the levels of the dominant component (at a given frequency) is equal in both signals.
  • the audio signal enhancing device when used in conjunction with a centre channel processor, results in a centre channel audio signal without any time-smearing which closely follows the input signal's level and reproduces the original sound image with fidelity.
  • the adaptive filters align both the phase and magnitude of components in the input signals so that when the filtered signal is summed with the non-filtered signal, a summed signal is produced with minimal time-smearing artefacts and comprising a high ratio of correlated components to non-correlated components.
  • the audio signal enhancing device when used in conjunction with an LFE channel processor, results in a subwoofer audio signal where, since only the loudest level of two filtered signals is output, out of phase signals are not cancelled and the resulting level of the output channel is proportional to the original low frequency content in the original input signals.
  • the enhancing device when used in combination with a centre channel processor or LFE processor, results in improved centre channel and LFE signals wherein the problems of the prior art have been resolved.
  • the centre and LFE signals contain a stable, non time-smeared image with a high quality natural sounding fidelity.
  • a front channel processor 106 comprises an audio signal enhancing device 201 as depicted in FIG. 2A .
  • the enhancing device 201 comprises an intermediate processor 202 and a control processor 203 .
  • the intermediate processor 202 in conjunction with the control processor 203 , processes the first input signal 102 and the second input signal 104 to yield at least one enhanced signal 204 a to 204 c.
  • the front channel processor 106 comprises the audio signal enhancing device 201 in combination with a centre channel processor 205 .
  • the at least one enhanced signal 204 may be further processed by the centre channel processor 205 to yield a centre channel output signal 206 .
  • the front channel processor 106 comprises the audio signal enhancing device 201 in combination with a LFE processor 207 .
  • the at least one enhanced signal 204 may be further processed by the LFE processor 207 to generate a single subwoofer signal 208 c .
  • a plurality of these enhanced signals 204 may also be further processed by the LFE processor 207 to generate at least three output signals, a first LFE signal 208 a , a second LFE signal 208 b , and a third LFE centre signal 208 c.
  • the front channel processor 106 comprises the audio signal enhancing device 201 in combination with a centre channel processor 205 and LFE processor 207 .
  • the at least one enhanced signal 204 may be further processed by the LFE processor 207 to generate a centre channel signal 206 and a single subwoofer signal 208 c , or a plurality of subwoofer signals 208 a , 208 b and 208 c.
  • the novel enhancing device 201 enables a high quality non-time smeared centre channel and at least one high quality special effects LFE channel to be generated respecting the original input signal fidelity enhanced with stable high quality subwoofer effects.
  • intermediate processor 202 and control processor 203 may be separate components or may form part of a single processor.
  • the control processor may also be a dedicated processor for controlling the operations necessary for generating the improved centre and LFE channels, or it may be a general purpose processor part of a broader upmixing system, which has tasks assigned to it of controlling the operations necessary for generating the improved centre and LFE channels.
  • the invention provides methods and devices that implement various aspects, embodiments, and features of the invention, and are implemented by various means. For example, these techniques may be implemented in hardware, software, firmware, or a combination thereof.
  • the various different means or configurations for implementing the features of the invention may be embodied as components, modules, apparatus or systems. For example, for the case of a component, it may implement a process running on a processor, a processor, an object, an executable, a thread of execution, a program, and/or a computer.
  • an application running on a computing device and the computing device can be a component.
  • One or more components can reside within a process and/or thread of execution and a component may be localized on one computer and/or distributed between two or more computers.
  • these components can execute from various computer readable media having various data structures stored thereon.
  • a memory can be configured to retain and a processor can be configured to execute instructions relating to the functions and method steps of the invention.
  • FIG. 3 depicts in further detail the audio signal enhancing device 201 according to one aspect of the present invention.
  • the enhancing device 201 comprises an intermediate processor 202 and a control processor 203 .
  • the intermediate processor 202 comprises a cross-talk stage 301 wherein a portion of the first input signal 102 is weighted using a gain coefficient gC 1 and combined with the second input signal 104 yielding a third signal 302 .
  • a portion of the second input signal 104 is weighted using a gain coefficient gC 2 and combined with the first input signal 102 yielding a fourth signal 304 .
  • two parallel processing lines are opened, each processing line comprising two processing branches.
  • the first processing line includes a first processing branch comprising component 318 and a second processing branch comprising components 306 and 310 .
  • second processing line includes a first processing branch comprising component 3320 and a second processing branch comprising components 308 and 312 .
  • third signal 302 is weighted by gain coefficient gD 1 306 and delayed in delay line 310 to yield a first delayed signal 314 .
  • fourth signal 304 is weighted by gain coefficient gD 2 308 and delayed in delay line 312 to yield a second delayed signal 316 .
  • third 302 and fourth 304 signals are filtered by first adaptive filter 318 and second adaptive filter 320 , respectively, to yield a first adapted signal 322 and a second adapted signal 324 , respectively.
  • the first adapted signal 322 is combined with the second delayed signal 316 in combiner 326 to yield first summed signal 340 .
  • first summed signal 340 and second summed signal 342 are each weighted by gain coefficients g 1 and g 2 respectively, thereby generating first 346 a and second 346 b enhanced signals.
  • First and second enhanced signals are then combined in combiner 344 generating enhanced signal 346 c . At least one of these enhanced signals 346 is used as input to centre channel processor 205 and/or LFE channel processor 207 , depending on the final configuration or implementation.
  • FIG. 3 also depicts the control processor 203 which is in communication with the various modules of the intermediate processor 202 and performs various analysis, monitoring, controlling and parameter setting operations as it uses the analysis results of various signals in order to achieve different advantageous effects.
  • Control processor 203 analyzes at least one of the original input signals 102 or 104 , at least one of the adaptive filter vectors AF_LS or AF_RS from first 318 or second adaptive filter 320 , or at least one of the first and second summed signals from summing units 326 and 328 .
  • the gain coefficients gC 1 and gC 2 of the cross-talk stage of the intermediate processor 202 are set in a first step by the control processor 203 to control how much one signal is added to the other in order to maintain the fidelity of the original signals.
  • control processor determines the amplitude and phase of each input signal and sets the gain coefficients accordingly, so that the end listener will have a natural experience.
  • the value of gC 1 and gC 2 which determines the degree of added cross-talk, is dependent on the level of the input signal correlation or the level difference (“level” here applies to a relative voltage magnitude, e.g. level in dBV) between the input signals.
  • Correlation between two signals can be measured as the average cross-correlation between two input signal buffers, or as the maximum value over a given lag, for example, ⁇ 100 ms.
  • the correlation can be estimated from the magnitude of the adaptive filter tap coefficients. That is, for the case where the input signals are essentially uncorrelated the magnitude of the adaptive filters (for example, for a given tap of the filter frequency vector) will be essentially zero.
  • gC 1 and gC 2 are increased to a maximal value (e.g. ⁇ 5 dB) when the input signals are highly uncorrelated (for example, when the running correlation is between ⁇ 0.1 and 0.1) or when there is a large inter-channel level difference, for example, with an absolute level difference greater than 15 dB.
  • a maximal value e.g. ⁇ 5 dB
  • the input signals are highly uncorrelated (for example, when the running correlation is between ⁇ 0.1 and 0.1) or when there is a large inter-channel level difference, for example, with an absolute level difference greater than 15 dB.
  • gC 1 and gC 2 are equal to a value of approximately ⁇ 30 dB for highly correlated signals (for example, when the absolute value of the running correlation is above 0.9), or when the inter-channel level difference is small, for example, with an absolute level difference less than 5 dB.
  • the gain coefficients of the delay lines gD 1 and gD 2 are set by the control processor 203 to control the ratio of correlated signal over uncorrelated signals.
  • the value of gain gD 1 306 may be identical or different to gain gD 2 308 depending on the characteristics of the intermediate output signal 346 desired.
  • the magnitude of these gains affects how much of the original input signals are summed with the signals filtered in the parallel adaptive filter lines. Since non-correlated information of the original signal are mixed with correlated components of the original signal that have been amplified by the adaptive filters, the gain acts as a control for the relative ratio of correlated versus non-correlated information that may appear at the output of the intermediate processor. In a first step the degree of correlation is ascertained, and in a second step and the gain and adaptive filter coefficients are subsequently set by the control processor 203 so that the delayed signals and the filtered signals are eventually matched.
  • both gains 306 and 308 are the same and both delay lines 310 , 312 apply the same delay.
  • control processor 203 updates the coefficients of the adaptive filters so as to both minimize the level of the difference output signal and the correlation between the output signal and input signal.
  • Implementing the NLMS in the frequency domain has the advantage that it is computationally less complex, however it may also be implemented in the time domain.
  • the adaptive filter is adjusted over time so as to decrease the error signal level.
  • the requirement for the algorithm is to determine the operating conditions for which J attains its minimum value.
  • This state of the adaptive filter is called the “optimal state”.
  • the rate of change in the error signal level (that is J) with respect to the filter coefficients h will be minimal.
  • This rate of change (or gradient operator) is an M-length vector r, and applying it to the cost function J gives:
  • Updating the filter vector h from time sample (n ⁇ 1) to time (n) is done by multiplying the negative of the gradient operator by a constant scaler and the filter update (i.e. the steepest descent gradient algorithm) is:
  • h ⁇ ( n ) h ⁇ ( n - 1 ) + ⁇ ⁇ + x T ⁇ ( n ) ⁇ x ⁇ ( n ) ⁇ x ⁇ ( n ) ⁇ e ⁇ ( n ) ⁇ ⁇ with ⁇ ⁇ 0 ⁇ ⁇ ⁇ 2.
  • delta is a regularization constant to ensure against computational errors when the power estimate of the input signal is too low (this update version is called the Normalized LMS algorithm).
  • the performance of the frequency domain and time domain NLMS algorithm are equivalent.
  • the overlap-save technique can be used with an overlap factor of two or four.
  • the time-domain constraint (to ensure against “wrap-around” errors when M is less than the length of the actual impulse response) can be affected so as to weight later coefficients less than early ones; a modification known as the “exponential step” (ES) algorithm. This ensures an exponential decay of the impulse response.
  • gain coefficients g 1 and g 2 are set by the control processor 203 to a value of unity.
  • the first and second enhanced signals are fed to the third combiner in equal proportions.
  • gain coefficients g 1 and g 2 are set by the control processor 203 .
  • the control processor 203 analyses the input signals 102 and 104
  • gain coefficient g 1 is set to a large value and gain coefficient g 2 to a low value when the first input signal level is larger than the second input signal level (and vice versa) in order to amplify the strongest of the enhanced signals.
  • the control processor 203 analyses the output of the adaptive filters
  • gain coefficient g 1 is set to a large value and gain coefficient g 2 is set to a low value when the relative phase of the adaptive filters differs by more than a predetermined amount, for example, 10 degrees phase angle. This configuration prevents distortion and time-smearing amongst the enhanced signals by keeping the phase differences within a predetermined range.
  • g 1 and g 2 are set to equal values, for example 0.5, but at least one adaptive filter is modified so that the relative phase of the two filters are equal. This can be achieved either by modifying the filter taps so that the imaginary component of one filter is shifted so that it matches the other filter, or by averaging the phase of both filters, or by a time-domain operation, whereby the peak of the time domain filter is shifted.
  • the group delay of the adaptive filters would be modified such that the first 340 and second 342 summed signals are time-aligned at the input of the summer 344 thereby generating a non time-smeared intermediate output signal 346 .
  • control processor comprises logic for determining the point at which the control processor changes state, for example, from a first state where the first summed signal 340 has the highest signal level to a second state where the second summed signal 342 has the highest signal level.
  • control processor slowly changes the gain of the two gain coefficients g 1 and g 2 for instance with such a time constant that it takes 500 ms to fade from one summed signal to the other. This gradual adjustment allows a smooth adjustment of sound contributions in the different channels, without interrupting the listening experience for the end user as well as minimising any distortion artefacts due to rapid gain changes.
  • control logic comprises a hysteresis system to limit the minimum time interval at which the control logic changes state, which in one embodiment is 500 ms, as depicted in the process 900 of FIG. 9 , which will be explained in further detail with reference to the preferred embodiments of the invention.
  • the combination of the intermediate processor 202 and control processor 203 yields various advantages by generating enhanced intermediate signals by taking into account the common sound components between the input signals, as the configuration of adaptive filters and delay lines, together with the dynamic setting of gain coefficients, allows the correlated components of the input signals to be utilised and tuned according to the desired effect.
  • the enhancing device mixes only the loudest level (“level” here applies to a relative voltage magnitude, e.g. level in dBV) of two filtered signals so that out of phase signals are not cancelled, and the resulting level of the output channel is proportional to the original low frequency content in the original input signals. This is achieved in part by determining a pair of optimum filters that are used to filter two input signals so that when summed, the resulting signal will not contain time-smearing and the dominant component (at a given frequency) is equal in both signals.
  • FIG. 4 depicts an embodiment of the process 400 for generating an enhanced signal 204 according to the present invention.
  • the process 400 is represented as functional blocks, which may be implemented by various means. For example, these techniques may be implemented in hardware, software, firmware, or a combination thereof.
  • the left hand column of functional blocks may be considered to be a first parallel processing line whereas the right hand column of functional blocks may be considered to be a second parallel processing line.
  • two original input signals 102 , 104 corresponding to a first and second audio signal are received in block 402 and block 403 respectively.
  • the two original input signals are each respectively processed by a cross-talk stage, in blocks 404 and 405 , to combine a portion of the second signal 104 to the first signal 102 to generate a first cross-talk signal 302 , and to combine a portion of the first signal 102 to the second 104 to generate a second cross-talk signal 304 , where the level of the cross-talk component is determined by gain coefficients gC 1 and gC 2 , wherein gC 1 ⁇ 1 and gC 2 ⁇ 1.
  • the first crosstalk signal 302 is modified, in block 406 , with gain gD 1 306 (where gain gD 1 can be equal to any value between zero and unity) and delayed, in block 408 , with a first delay unit 310 , which in one embodiment of the invention is a delay equal to 10 ms, to generate a first delayed signal 314 .
  • the second crosstalk signal 304 is modified, in block 407 , with gain gD 2 308 and delayed, in block 409 , with second delay unit 312 to generate a second delayed signal 316 .
  • the first crosstalk signal 302 is filtered, in block 410 , using a first adaptive filter 318 to generate a first adapted signal 322 and the second crosstalk signal 304 is filtered, in block 411 , using a second adaptive filter 320 to generate a second adapted signal 324 .
  • the first adapted signal 322 is combined, in block 412 , with the second delayed signal 316 to generate a first summed signal 340 . If gain gD 2 is set to zero, then summing unit 326 directly passes the signal from filter 318 .
  • the second adapted signal 324 is combined, in block 413 , with the first delayed signal 314 to generate a second summed signal 342 . Again if gain gD 1 is set to zero, then summing unit 328 directly passes the signal from filter 320 .
  • a first gain coefficient g 1 is applied to the first summed signal 340 to generate first enhanced signal 420 a .
  • a second gain coefficient g 2 is applied to the second summed signal 342 to generate a second enhanced signal 420 b .
  • Both of these enhanced signals are finally combined in combiner 344 to generate a third enhanced signal 420 c .
  • These enhanced signals are used in combination with the centre channel processor 205 and LFE channel processor 207 to achieve the upmixed output signals of the present invention.
  • the filter coefficients of the first 318 and second 320 adaptive filters are also updated as previously explained.
  • the process 400 yields at least one enhanced signal 420 which enables a high quality non-time smeared centre channel and at least one high quality special effects LFE channel to be generated respecting the original input signal fidelity enhanced with stable high quality subwoofer effects.
  • the outputs A, B and C of this process 400 are linked to process 700 and process 900 for generating the centre channel signal and the at least one subwoofer channel signal.
  • FIG. 5 depicts a preferred embodiment of the invention in an upmixing system for generating a centre channel signal exhibiting the advantages of the present invention, and it corresponds to a detailed view of FIG. 2B , wherein the detailed elements of intermediate processor 202 of FIG. 3 have also been depicted.
  • control processor 203 takes as input the input signals 102 and 104 , and outputs, amongst other parameters, the gain coefficients gC 1 , gC 2 , gD 1 , gD 2 , adaptive filter coefficients as well as gain coefficients g 1 , g 2 .
  • the third enhanced signal 346 c is input into centre channel processor 205 .
  • Centre channel processor 205 comprises a processor for determining the dominant image direction 501 followed by a centre channel weighting processor 503 .
  • the dominant image direction processor 501 accepts as input information from at least one of the adaptive filters 318 and 320 , or by analysis of the input signals Lo 102 and Ro 104 .
  • the dominant direction may be determined using only one adaptive filter. In such case the level of just one filter relative to unity is used to determine the dominant direction. However, when only one filter is used, the dominant direction is calculated as the absolute energy level within a given frequency band for that filter. This method is not ideal as there may be zero signal energy at a given frequency in one channel, but a non-zero level in the other channel, and in such cases the dominant signal would be calculated incorrectly.
  • the dominant direction is calculated as a level ratio of the two filters that can be operated in the frequency domain or band-limited time-domain, or in other words, as the average of the filter coefficients of both adaptive filters, thereby reducing the risk of incorrect calculation and increasing the quality of the dominant image direction determination.
  • the dominant image direction can also be calculated in a similar way by analysis of the original input signals.
  • centre channel weighting coefficient CCWC, processor 503 , also known as spatial filter, where a coefficient for the intensity of the centre channel is determined.
  • a high valued coefficient corresponds to a direction in a central location, which in one configuration is determined when the two adaptive filter coefficients AF_LS and AF_RS have essentially equal values (for example, the magnitude of the nth tap in a frequency domain representation of the both filters has the same value).
  • This formula may also be expressed as the maximum value between zero and the cosine of the average magnitude of the filter coefficients of both adaptive filters, divided by a constant C, with the cosine value raised to the power of N.
  • Constant C likewise controls the spatial width for the centre channel, however does not change the shape of the spatial filter.
  • d_wt may be the absolute value of a single adaptive filter, in which case a CCWC value may be calculated twice, once per adaptive filter. The final CCWC weighting coefficient would then be determined as the average of these two intermediate CCWC values.
  • FIG. 6 depicts a curve showing how the centre channel weighting coefficient is affected by the determined image direction. If the image direction is determined to be essentially equal to the direction of the physical loudspeaker, which in one configuration is determined when the magnitude of one adaptive filter is 20 dB greater than the other (which can occur if a sound source is hard-panned to one channel by a mixing engineer), then the centre channel weighting coefficient is set to a value substantially equal to zero. This ensures that for such “hard panned” instances, the output level of the centre channel will be zero, and the dominant image direction will be perceived as located in the direction of a single front left or right loudspeaker.
  • the image direction is determined to be essentially equal to zero degrees (that is, the CCWC value is set to be equal to its maximum value) if speech is detected in the intermediate signal 346 .
  • the determined centre channel weighting coefficient CCWC is multiplied in multiplier 505 by the third enhanced signal 346 c from the intermediate processor 202 .
  • the signal generated is the centre channel signal 206 ready to be applied to a suitable transducer such as a loudspeaker.
  • Multiplier 505 may be implemented in the time domain or frequency domain in a manner well known to the person skilled in the art. As an example, multiplier may be implemented in the time domain as a convolution operation or in the frequency domain by frequency-dependent filters.
  • a negative gain 507 may be optionally applied, that in one configuration is equal to a 3 dB attenuation, to compensate for this increase, to generate a modified output centre channel signal 346 c.
  • the adaptive filter coefficients, AF_LS and AF_RS, the gains g 1 and g 2 , the determined dominant image direction and centre channel weighting coefficients CCWC can be represented as vectors having a single value or having a frequency-dependant representation (that is, for a frequency-dependant representation there are different vector values for different frequencies).
  • to generate the centre channel signal of the present invention involves at least the steps of combining the adaptive filtered input signals generated from two input signals to generate two combined signals, which are mixed to generate a third summed signal, this mixing may be implemented in varying proportions, and finally the third summed signal is weighted by a vector CCWC that considers the dominant direction of the front image, whereby if the dominant direction is determined to be substantially equal to zero (that is, the direction of the centre speaker) then the CCWC is high, and if the absolute value of the dominant direction is determined to be high then the CCWC is a low value.
  • this novel method for generating a centre loudspeaker channel is that the adaptive filters align both the phase and magnitude of components in the input signals so that when the filtered signal is summed with the non-filtered signal, a summed signal is produced with minimal time-smearing artefacts and an increase in the ratio of correlated components to non-correlated components (that is, those components in the original input signals 102 , 104 that are positively correlated).
  • a centre channel signal is generated which contains a stable non time-smeared image with a high quality natural sounding fidelity.
  • audio input test signals are used that are typical for music, movie sound-track, and commercial voice audio.
  • the Ro input signal has a 3 dB boost and 0.5 ms advance relative to the Lo input signal, and that the Lo and Ro signals are correlated, such as would occur for a spaced 2-microphone recording or a single sound source, with the sound source closer to one microphone than the other, where the output of one microphone is the Lo signal and the output of the other microphone is the Ro signal.
  • the second adaptive filter 320 will try to align these two signals by applying a 3 dB gain and 0.5 ms advance (that is, assuming that the delay of the Ro signal is greater than 0.5 ms, then this means that the time-domain peak in the second adaptive filter 320 will be such that the Lo channel is effectively advanced relative to the Ro signal).
  • the first adaptive filter 318 will have an inverse response to the second adaptive filter 320 , that is a magnitude of ⁇ 3 dB, and will have a time-domain peak in the first adaptive filter 318 such that the Ro channel is effectively delayed relative to the Lo signal.
  • the resulting signal level of the Lo signal filtered with the second adaptive filter 320 will be +3 dBV (we are also assuming that the cross-talk level set by gain gC 1 is low, for example, ⁇ 15 dB).
  • the filtered Lo signal will also be time-shifted by 0.5 ms to align with the Ro signal, generating a new first summed signal.
  • the second Ro signal is processed with the ⁇ 3 dB second adaptive filter 320 and summed with the delay first Lo signal giving a second summed signal with a level of approximately 0 dB.
  • the first adaptive filter 318 will have a ⁇ 0.5 ms delay
  • the second summed signal will be delayed by 0.5 ms relative to the first summed signal.
  • the centre channel weighting coefficient that is then applied to the centre channel is calculated from the level difference between the two channels. This can be calculated using one of, or both, of the frequency-dependant level differences between the two input signals or the level difference between the first 318 and second 320 adaptive filters.
  • the max( ) function returns the maximum value of the cos( ) function and zero, that is, bounding CCWC to a value between zero and unity.
  • a further gain reduction is applied to the summed signal from summer, applying a further gain, that is approximately equal to a 3 dB attenuation (this accounts for the fact that summed partially coherent data sequences give a level increase of approximately 3 dB).
  • the centre channel is slightly softer than the level of the right channel (which has a +3 dB level for the portion under consideration, compared with a 0 dB level for the left channel). Therefore a perceptual sound image would be localized between the centre and right loudspeaker signal.
  • Modifying the exponent value N in the above CCWC formula would modify the “sharpness” of the CCWC, that is, smaller value exponent increases CCWC as a function of abs(d_wt), so the centre channel level is higher for sources that are nearly hard-panned, giving a sound image that is localized closer to the centre loudspeaker.
  • Changing the value of the exponent can be considered a divergence control controlling how much a mono or nearly-mono original input signal is sent to the centre channel relative to the front left and right channels of the upmixed audio system. This has the advantage that a user can control the sensitivity of the centre channel according to personal preferences.
  • FIG. 7 is a flowchart representation of a process 700 for generating the centre channel signal.
  • FIG. 7 represents amongst others also the steps taken by control processor 203 in performing various analysis, monitoring, controlling and parameter setting operations.
  • the process 700 is represented as functional blocks, which may be implemented by various means. For example, these techniques may be implemented in hardware, software, firmware, or a combination thereof.
  • the process starts by determining 704 the dominant image direction and determining 706 the central channel weighting coefficient as explained earlier.
  • the third enhanced signal 346 c of FIG. 3 or FIG. 5 is received as depicted by circle C (corresponding to output circle C of process 400 of FIG. 4 .
  • the third enhanced signal 346 c is multiplied 708 by the determined CCWC and attenuated 710 by attenuation coefficient in order to yield 712 the final centre channel output signal 206 .
  • the centre channel weighting coefficient is a result of calculating the magnitude of the first and second adaptive filters modified by a direction weighting component.
  • the output is the output signal for the centre channel 206 ready to be applied to a suitable transducer such as a loudspeaker. Since the summing of partially coherent data sequences results in a level increase of approximately 3 dB, a further gain may be optionally applied 708 , that in one embodiment is essentially equal to a 3 dB attenuation, to compensate for this increase, to generate a modified output centre channel signal exhibiting the advantages of the present invention.
  • the audio signal enhancing device when used in conjunction with a centre channel processor, results in a centre channel audio signal without any time-smearing which closely follows the input signal's level and reproduces the original sound image with fidelity.
  • the adaptive filters align both the phase and magnitude of components in the input signals so that when the filtered signal is summed with the non-filtered signal, a summed signal is produced with minimal time-smearing artefacts and comprising a high ratio of correlated components to non-correlated components.
  • FIG. 8 depicts another embodiment of the invention in an upmixing system for generating at least one LFE subwoofer audio signal exhibiting the advantages of the present invention, and it corresponds to a detailed view of FIG. 2C , wherein the detailed elements of intermediate processor 202 of FIG. 3 have also been depicted.
  • the configuration allows for only one subwoofer LFE signal 208 c to be generated, it also allows for three subwoofer LFE signals 208 to be generated, comprising a first LFE1 208 a , a second LFE2 208 b and a third centre LFEc 208 c subwoofer channel.
  • control processor 203 takes as input the two signals 102 and 104 , and outputs, amongst other parameters, the gain coefficients gC 1 , gC 2 , gD 1 , gD 2 , adaptive filter coefficients as well as gain coefficients g 1 , g 2 .
  • the Lo 102 and Ro 104 input signals are first processed by low pass filters 801 , 803 , LPF, each, before being analyzed by the control processor 203 so that the level analysis performed by the control processor only takes the low frequency energy content into consideration.
  • the LFE channel processor 207 acts on different points of the intermediate processor 202 .
  • the third LFEc channel 208 c is generated by low pass filtering the third enhanced signal 807 .
  • the LFE1 channel 208 a is generated by low pass filtering the second enhanced signal 809 resulting from the application of gain coefficient g 2 to the second summed signal 342 .
  • the LFE2 channel 208 b is generated by low pass filtering the first enhanced signal 809 resulting from the application of gain coefficient g 1 to the first summed signal 340 .
  • Each of these output signals can be reproduced with a subwoofer loudspeaker device allowing for a multi-subwoofer configuration as is found in some theatre systems.
  • Low pass filtering may be implemented in the digital domain, such as using digital finite impulse response FIR filters, or infinite impulse response IIR filters, or in the analogue domain.
  • the cut-off frequency can be controlled by a user interface or set automatically, for instance with a ⁇ 3 dB cut-off frequency of 75 Hz.
  • Control processor may also perform the low pass filtering by setting the filter coefficients internally to undertake a low frequency weighting.
  • the third LFEc signal 208 c can be used, as this contains components of both the original left 102 and right 104 input signals.
  • FIG. 9 is a flowchart representation of a process 900 for generating at least one LFE subwoofer signal.
  • FIG. 9 represents amongst others also the steps taken by control processor 203 in performing various analyses, monitoring, controlling and parameter setting operations.
  • the process 900 is represented as functional blocks, which may be implemented by various means. For example, these techniques may be implemented in hardware, software, firmware, or a combination thereof.
  • the process starts by first low pass filtering 904 , 905 , LPF, each received 902, 903 input signal.
  • Control processor 203 subsequently analysis the levels of the low pass filtered signals by calculating 906 , 908 the levels of two different signals. In step 908 a comparison is made to determine which of the two signals has a higher level and the control processor 203 acts to keep the loudest of the enhanced signals and discard the weakest of the enhanced signals.
  • the discarding of the weakest signal is not performed abruptly, but as a slow fade.
  • the first gain coefficient g 1 is calculated as the last updated coefficient g 1 multiplied by a parameter mu
  • the second gain coefficient g 2 is calculated as the last updated coefficient g 2 multiplied by unity minus the parameter mu.
  • L 2 has a higher level than L 1 the roles are reversed and the first gain coefficient g 1 is calculated as the previous coefficient g 1 multiplied by unity minus a parameter mu, and the second gain coefficient g 2 is calculated as the previous coefficient g 2 multiplied by the parameter mu, where parameter mu>1.
  • both gain coefficients are applied to the combiners of FIG. 3 to yield the signals 805 , 807 and 809 which are subsequently low pass filtered to be reproduced with a subwoofer loudspeaker device allowing for a multi-subwoofer configuration as is found in some theatre systems.
  • Control processor 203 determines the levels of the two input signals and sets the gain coefficient g 1 to a large value and g 2 to a low value depending on which of the two input signals is determined to have a larger signal level. This ensures that when there is an out-of-phase low frequency component in the original left and right input signals (as a result of a common audio mixing technique), the summation of the first and second summed signals will not cancel the out-of-phase low frequency component.
  • the audio signal enhancing device, and corresponding method when used in conjunction with an LFE channel processor, results in a subwoofer audio signal where, since only the loudest level of two filtered signals is output, out of phase signals are not cancelled and the resulting level of the output channel is proportional to the original low frequency content in the original input signals.
  • the devices and methods of the present invention provide a variety of advantageous characteristics, amongst them the enhancement of a stereophonic audio signal comprising two signals into at least one enhanced signal wherein out of phase signals are not cancelled, and the resulting level of the output channel is proportional to the original low frequency content in the original input signals.
  • the resulting signal will not contain time-smearing and the dominant component (at a given frequency) is equal in both signals, and the level of the new dominant signal has the same level as in the original two input signals.
  • centre channel processor when applied to the centre channel processor generates a centre channel signal comprising a balanced dominant component without any time-smearing which closely follows the input signal's level with minimal time-smearing artefacts and comprising a high ratio of correlated components to non-correlated components.
  • this enhanced signal when applied to the low frequency effects processor generates at least one subwoofer signal wherein out of phase signals are not cancelled and the resulting level of the output channel is proportional to the original low frequency content in the original input signals.
  • a plurality of LFE signals may also be generated from the plurality of enhanced signals generated by the audio signal enhancing device of the present invention.
  • the embodiments described herein may be implemented by hardware, software, firmware, middleware, microcode, or any combination thereof.
  • systems and/or methods are implemented in software, firmware, middleware or microcode, program code or code segments, a computer program, they may be stored in a machine-readable medium, such as a storage component.
  • a computer program or a code segment may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements.
  • a code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted using any suitable means including memory sharing, message passing, token passing, network transmission, etcetera.
  • the techniques described herein may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein.
  • the software codes may be stored in memory units and executed by processors.
  • the memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor through various means as is known in the art.
  • at least one processor may include one or more modules operable to perform the functions described herein.
  • various aspects or features described herein may be implemented as a method, apparatus, or article of manufacture using standard programming and/or engineering techniques.
  • article of manufacture as used herein is intended to encompass a computer program accessible from any computer-readable device, carrier, or media.
  • computer-readable media can include but are not limited to magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips, etc.), optical disks (e.g., compact disk (CD), digital versatile disk (DVD), etc.), smart cards, and flash memory devices (e.g., EPROM, card, stick, key drive, etc.).
  • various storage media described herein can represent one or more devices and/or other machine-readable media for storing information.
  • machine-readable medium can include, without being limited to, various media capable of storing, containing, and/or carrying instruction(s) and/or data. Additionally, a computer program product may include a computer readable medium having one or more instructions or codes operable to cause a computer to perform the functions described herein.
  • DSP digital signal processor
  • ASIC application specific integrated circuit
  • FPGA field programmable gate array
  • a general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine.
  • a software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art.

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